A typical organic electroluminescent element (hereinafter also referred to as “organic EL element”) is composed of a cathode, an anode, and a luminous layer disposed therebetween and containing a luminous compound. Such an organic EL element can emit light by the following mechanism: An electric field applied to the organic EL element recombines holes injected from the anode with electrons injected from the cathode in the luminous layer to generate excitons, which are deactivated with luminescence (fluorescence and/or phosphorescence). The organic EL element is a completely solid element that includes submicron films disposed between the electrodes and composed of organic materials, and can emit light under an applied voltage of several volts to several tens of volts. Based on these advantages, it is expected that the organic EL elements will be applied to flat displays and lighting in the next generation.
Since Princeton University reported an organic EL element by phosphorescence from the excited triplet (for example, see Non-Patent Document 1), phosphorescent materials at room temperature have been extensively studied (for example, see Patent Document 1 and Non-Patent Document 1) to develop organic EL elements for practical use.
It has been already found that such phosphorescent compounds or complexes emit light beams of different color tones for various uses, that is, light beams of blue (B), green (G), and red (R) by varying their chemical structures, such as trisphenylpyridine iridium complexes described in J. Am. Chem. Soc., vol. 107, p. 1431 (1985), tris(phenylisoquinoline) iridium complexes described in J. Am. Chem. Soc., vol. 125, p. 12971 (2003), and tris(phenyltriazole) complexes described in Chem. Mater., vol. 18, p. 5119 (2006).
These phosphorescent complexes have their own emission spectra according to a difference in chemical structure. Unfortunately, original luminous colors unique to the respective chemical structures of the complexes are often not achieved due to agglomeration and/or crystallization of the complexes, which shifts the spectra to longer regions with broader distributions. To avoid such agglomeration and/or crystallization, the phosphorescent complexes are often dispersed in binders, or are used in combination with host compounds. These countermeasures, however, are not sufficiently effective, and still cause remarkable changes in color tone in the phosphorescent complexes during long-term use or at high temperatures.
The agglomeration and/or crystallization of the phosphorescent complex is fundamentally caused by the intensity of the interaction energy of the phosphorescent complex. The interaction balance with a co-existing host compound determines the state of the phosphorescent complex in a film, and this state varies over time to change the intensity of the interaction energy of the phosphorescent complex.
Such a disadvantage can be solved by formation of a stable film. The state of the film can be stabilized by a large negative Gibbs free energy of the film.
The Gibbs free energy is determined by enthalpy and entropy according to the second law of thermodynamics. Enthalpy is largely determined by a chemical structure intrinsic to a complex molecule, and cannot be readily varied. Entropy is determined by the number and distribution of components, and can be used as a universal technical variable factor.
This is rationally described from an entropy effect.
The entropy effect will be described with reference to the diagrams. FIG. 1A and FIG. 1B are a schematic view explaining an increase in entropy by mixing of two components. FIG. 1A illustrates a model of mixing of components A with components B. FIG. 1B is a model of mixing of components A with components A.
A reaction at constant pressure and low temperature has the following relationship among a change in Gibbs free energy (ΔG), a change in enthalpy (ΔH), and a change in entropy (ΔS), which is represented by following Expression (1) where T represents absolute temperature.ΔG=ΔH−TΔS  Expression (1):
For example, assume that 2n phosphorescent organic metal complex molecules (component A) are present in a film. Assume that the film originally has a half of the 2n complex molecules (i.e., n complexes), and the other n complex molecules (component A) of the same type are added in the film so that the total number is 2n and the volume is doubled. At this time, the entropy change is zero because the complex added is the same as that originally present in the film (FIG. 1B). In contrast, if n complex molecules of a different type (component B) are added, the entropy of the complex originally present in the film (component A) increases because of the added complex molecules (component B) (FIG. 1A). This increase in entropy refers to an entropy effect. The increase in entropy causes the Gibbs free energy in the film to be more negative for stabilization, attaining a stable film over time. The entropy effect acts on such a basic principle.
This entropy effect caused by the “complex molecules of a different type” can attain both the stability of the film and interactive deactivation of it-planes to effectively prevent agglomeration over time of the complex mixture.
This phenomenon is found not only in the films but also in solutions of complexes. Namely, the schematic view in the right of FIG. 1A is also considered to illustrate a mixed solution of isomeric complexes, the isomeric complexes being completely dissolved or separately dispersed in a solvent. The Gibbs free energy of the solution or the thin film is negatively larger than that of a solution or powder composed of a single complex (corresponding to the right diagram in FIG. 1B), and changes caused by disturbance is reduced. In other words, agglomeration and/or recrystallization of the complexes is prevented.
This entropy effect attains dispersion of the complex during film formation by application of the solution of the mixed complexes to prevent changes in films over time and after electrical conduction. The entropy effect also enables the sublimation of the complex during sublimation purification or deposition, and sublimation of the complex in the form of a single molecule, enabling formation of films having a complex almost ideally dispersed (or nearly separately dispersed) therein.
An increase in entropy as described above is effectively attained by co-existing several luminous complexes. In co-existing several luminous complexes having different electronic states, however, electrons and holes (collectively referred to as charge carriers) are injected in different ways according to the types of complex molecules, and are preferentially injected into complex molecules to be most readily filled with charge carries. Such injection of charge carriers reduces the opportunity of recombination of charge carriers and in turn luminescence efficiency. This also shortens the luminance of the light emission over time and in turn the emission lifetime of the organic EL element.
The solution of such a problem requires mixing of as many compounds as possible, the compounds having different structures and having substantially the same level of the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO), and very close emission spectra and physical properties.
Such a requirement is satisfied by a heteroleptic complex (for example, see Patent Document 2) in which part of several ligands forming a complex is replaced with a ligand having a different structure. The heteroleptic complex moderates the crystallinity derived from a symmetric structure of a homoleptic complex to reduce growth of coarse crystals in the organic EL element. The heteroleptic complex, however, is readily agglomerated because the heteroleptic complex often has x-planes, which assist interaction between complex molecules and are present in the outermost region of the complex molecules.